1. Field of the Invention
This invention relates to a conductive roll for use in imparting a uniform charge to a photoconductor or the like of image forming machines, such as an electrophotographic copying machine and a printer, or a toner jet copying machine and a printer. The invention also relates to a method for producing the conductive roll.
2. Description of the Related Art
A conductive roll of an ionic conduction type, which has an alkali metal salt, such as lithium perchlorate, added to urethane, has so far been used as a cast urethane type conductive roll. However, it is highly environment-dependent in that its electrical resistance varies according to an environment, such as temperature and humidity. Thus, a conductive roll of an electronic conduction type, which has conductive carbon black added to urethane, is under consideration (see, for example, Japanese Patent Application Laid-Open No. 2003-202750 (claims, etc.)).
In the electronic conduction type conductive roll, uniform distribution of carbon black in urethane rubber is essential for obtaining a satisfactory image. During the forming of urethane, however, reaggregation of carbon black due to heat occurs along with a urethane reaction. Thus, differences arise in the distribution of carbon black in the urethane, causing nonuniformity to the electrical resistance value of the conductive roll. As a result, a printing product obtained by printing with the use of this conductive roll tends to have a poor image. Thus, it is necessary to cure urethane earlier than the reaggregation of carbon black, thereby containing the movements of carbon black so that a uniform distribution of carbon black will be obtained. For this purpose, the temperatures of the raw materials for urethane, the temperature of a mold for the forming of urethane, and a catalyst for use in the urethane reaction are changed, whereby the rate of the urethane reaction is controlled.
However, when the mold for use in the forming of urethane is brought to a high temperature, and urethane is formed at this high temperature, the viscosity of the raw materials for urethane decreases in the vicinity of the mold surface because of heat, accelerating the aggregation of carbon black. As a result, a uniform distribution of carbon black is not obtained. If the amount of the catalyst is increased, on the other hand, the reaction rapidly proceeds, starting in a stage where the raw materials for urethane are mixed, with the result that these raw materials thicken. Thus, when the raw materials are charged into the mold, bubbles tend to be entrapped, and urethane is cured unchanged, presenting a high possibility for occurrence of pinholes. In conclusion, it is very difficult to control the urethane reaction so that a uniform distribution of carbon black is obtained.